As a next generation semiconductor memory, a Phase-change Random Access Memory (PRAM) as a non-volatile memory using a phase-change technology has been developed. The PRAM uses a phase-change film as a memory element, where information is stored by changing a part of the film into a crystal state or an amorphous state. As the phase-change film, for example, a GST (germanium antimony tellurium: GexSbyTez) film is used. When heat corresponding to 600° C., which is the melting temperature of GST, or more is applied to GST in a short period of time (Treset<30 ns), and then the GST is rapidly cooled, the GST is in the amorphous state. When heat corresponding to a temperature less than the melting temperature is applied to the GTS in the amorphous state for a long time (Tset>about 50 ns), the GST is changed into the crystal state. As shown in FIG. 12, electric current is rarely changed with an applied voltage up to about 0.7 V, which is indicative of a large resistance, in the amorphous state, while electric current is increased in proportion to an applied voltage, which is indicative of a low resistance, in the crystal state.
Therefore, a heater film that controls a temperature of the GST film is provided to contact with the GST film. By adjusting current to be applied to the heater film and a period of time during which the current is applied, thereby adjusting Joule heat generated from the heater film, namely, by adjusting the temperature and the time period during which the GST film is heated, the GST film can be in the crystal state or the amorphous state. Specifically, after a current (melting current) sufficient to fully heat the GST film up to the melting temperature is applied to the heater film, the current is turned off, a memory cell is reset, while when a current (crystallization current) is applied to the heater film, and thus data are written into the memory cell. In addition, by applying to the GST film a current that exists in a current range corresponding to a high resistance in the amorphous state (high resistance) and a low resistance in the crystal state (low resistance), the data in the GST film can be read out. Here, when the current flows through the heater film, because the Joule heat is in proportion to the resistance of the heater film, the temperature of the GST film having higher resistance becomes higher at the same temperature.
As the heater film, a titanium nitride (TiN) film is considered as promising from a viewpoint of a cost and a thermal stability. However, because the TiN film used in the current semiconductor market has a low specific resistance of 130 μΩ•cm, when the TiN film is used to sufficiently heat the GST film, energy consumption is increased. When there is an attempt to reduce the energy consumption, the GST film cannot be sufficiently heated. In addition, since the TiN film has a greater specific resistance when oxidized, the GST film may be degraded when oxygen is diffused from a boundary between the TiN film and the oxide film because the GST film is not stable against oxygen. Moreover, when the TiN film is oxidized, a TiOx film, which is a robust insulating film, is formed. With this, a characteristic of the TiN film serving as metal is degraded. Therefore, use of the oxidized TiN film as the heater for heating the GST film is not advisable.
In addition, when the TiN film is thinned, the electric resistance of the TiN film becomes larger. However, it is difficult to deposit a thin film having greater film thickness uniformity across the surface thereof. Therefore, it has been desired to establish a method of generating greater Joule heat by increasing the specific resistance while the thickness of the TiN film is maintained at a certain thickness, for example, about 50 nm.
Incidentally, Patent Documents 1 and 2 describe a method of changing the specific resistance of the TiN film. Patent Document 1 discloses an invention that reduces impurities such as chlorine by exposing the TiN film to ammonia plasma, and Patent Document 2 discloses an invention that densifies the TiN film by exposing the TiN film to inert gas plasma, which reduce the specific resistance. When the TIN films are used as a diffusion preventive film against a plug filled in a via, the TiN films, which serve as a wiring structure, preferably have a low specific resistance. The invention disclosed in Patent Document 2 has been made from such a point of view.
Patent Documents 1 and 2 provide no idea of using the TiN film serving as the heater film for the phase-change film and no expectation of increasing the specific resistance. Therefore, even if the technologies disclosed in the documents are utilized, the specific resistance of the TiN film cannot be increased.
Patent Document 1: Japanese Translation of PCT International Patent Application No. 2001-508497.
Patent Document 2: Japanese Patent Application Laid-Open Publication No. H08-246152.